COTONOU, April 22 (Reuters) - Benin President Thomas Boni Yayi promised voters and world leaders including Barack Obama he would step down when his second term expires next year but doubts over his pledge are overshadowing Sunday's parliamentary election.

Lawmakers from the president's ruling alliance, the Cowry Forces for an Emerging Benin (FCBE), are trying to focus on jobs, roads and schools in the campaign, but his opponents portray the election as the only way to stop Boni Yayi clinging to power beyond the end of his mandate in 2016.

His plans to reform Benin's constitution - which would introduce a national electoral commission and state auditor to fight graft and ensure democratic elections - have fed the suspicions about the president's real intentions.

Constitutional reform has been a tool used by other African leaders to stay in office, though Benin is one of Africa's strongest democracies and its Constitutional Court has ruled that terms limits cannot be altered as part of the reforms.

"Boni Yayi can swear to whoever he wants but people don't believe him," said political analyst Davodoun Cyr. "He has created a crisis of confidence in the political class."

FCBE leaders predict it will win between 35 and 50 of the 83 seats in parliament - well short of the four-fifths majority needed for constitutional reform.

"But as the issue is sensitive in Africa and the opposition has no other arguments, they are clinging to it," said FCBE Secretary General Eugene Azatassou.

The opposition, which questions why the president is pushing the constitutional changes so late in his term, fear he aims to use the shake-up to restart the clock on his presidency.

Impromptu rallies and billboards around the bustling port city of Cotonou testify to the array of parties lined up against the ruling Cowry Forces for an Emerging Benin (FCBE).

For some in Cotonou's Houenoussou market, where stalls perch on the edge of rubbish-clogged drains, the president's energetic campaigning is a sign he has no intention of relinquishing power next year.

"I want the opposition to get a majority so we can ensure a change of team," said Marios Mele, a hotel manager.

A Cotonou-based diplomat said the proposed constitutional changes did not appear to be a bid by Boni Yayi to secure a third term: "But there is a lot of political mileage to be gained out of this."

Benin is forecast to post annual economic growth of five percent in coming years, driven by cotton exports and Cotonou's port, which feeds neighbouring Nigeria. But analysts say graft and poor infrastructure hamper Benin's development. (Editing by Daniel Flynn and Gareth Jones)